1. Field of the Invention
The present invention relates to a bumper reinforcement and a manufacturing method for the bumper reinforcement.
2. Description of the Prior Art
One of conventional bumper reinforcements will be described with reference to the drawings of FIG. 7, FIG. 8A, FIG. 8B, and FIG. 9.
As shown, a bumper reinforcement(a) is attached to a part of the vehicle body by means of a bumper facer (not shown), so that it is necessary to curve both end portions of the bumper reinforcement according to the curvature of the end portion of the bumper facer. The bumper reinforcement(a) has a sectional shape of a hat and is provided with a main body of the reinforcement(b) and flanges(e). When the bumper reinforcement(a) of such configuration is bent, cut-out portions(c) are formed at both ends of the reinforcement and outside material(d) is extended and squeezed inwardly.
The conventional bumper reinforcement(a) is made of high tension steel of 60kg or 80kg. Recently, 100kg or 120kg high tension steel has been used to make the reinforcements in order to make the product lightweight. Other bumper reinforcement having trough-like section have been made with similar methods.
According to the conventional manufacturing method, the bumper reinforcement(a) is manufactured by reduction and elongation. However, high tension steel plate has little elongation capability and is poor in resistance to buckling load. Accordingly, if the bumper reinforcement(a) is worked to obtain a hat-shape as shown in FIG. 8, the sectional shape of the product of reinforcement obtained is as shown in FIG. 9 having insufficiently sections indicated by the letter A. When curvature or diameter is small or bending angle is large, elongation percentage of high tension steel is too small to obtain products of the right shape. In practice, when the material is forcibly elongated, necking is induced and cracks are formed in the product.
In short, a material's percentage of elongation determines its range of formable shapes and makes it difficult to freely design its shape. In addition, when high tension steel must be shaped into a hat-shaped cross-section, a heavy duty or large press having a large workability is necessary.